Sputtering is a thin film coating process which involves the transport of almost any material from a target to a substrate of almost any other material. The ejection of the target material is accomplished by bombarding the surface of the target with gas ions accelerated by a high voltage. Particles of atomic dimension are ejected from the target as a result of momentum transfer between the accelerated gas ions and the target. Upon ejection, the target particles traverse the sputtering chamber and are subsequently deposited on a substrate as a thin film.
Sputtering processes utilize an enclosed chamber confining a sputtering gas, a target electrically connected to a cathode, a substrate, and a chamber which itself may serve as the electrical anode. A power supply is typically connected such that the negative terminal of the power supply is connected to the cathode and the positive terminal is connected to the chamber walls. In operation, a sputtering gas plasma is formed and maintained within the chamber near the surface of the sputtering target. By electrically connecting the target to the cathode of the sputtering power supply and creating a negative surface charge on the target, electrons are emitted from the target. These electrons subsequently collide with atoms of the sputtering gas, thus stripping away electrons from the gas molecules creating positively charged ions. The resulting collection of positively charged ions together with electrons and neutral atoms is herein referred to as a sputtering gas plasma. The positively charged ions are accelerated toward the target material by the electrical potential between sputtering gas plasma and target and bombard the surface of the target material. As ions bombard the target, molecules of target material are ejected from the target surface and coat the substrate.
One method of enhancing conventional sputtering processes is to arrange magnets behind or near the target to influence the path taken by electrons within the sputtering chamber, thereby increasing the frequency of collisions with sputtering gas atoms or molecules. Additional collisions create additional ions, thus further sustaining the sputtering gas plasma. An apparatus utilizing this enhanced form of sputtering by means of strategically located magnets is referred to herein as a magnetron system.
When attempting to form thin, electrically insulating layers by sputtering electrically insulating target materials such as various oxide (e.g. silicon dioxide) or nitride compounds, the insulative nature of such target materials prevents DC magnetron sputtering from occurring. However, it has been found that the insulating layer may be reactively formed on the substrate by sputtering the electrically conductive base element (e.g. silicon) with a reactive gas (e.g. oxygen or nitrogen). This approach of forming a compound film on the substrate is referred to herein as reactive sputtering.
One drawback encountered in sputtering processes (reactive and nonreactive) is the creation of sputtered regions and nonsputtered regions on sputtering targets. For example, in DC magnetron sputtering processes, sputtered regions arise from the particular arrangement of magnets near the magnetron sputtering cathode and target. For a planar target which has been at least partially sputtered, a sputtered region typically appears as an oval or racetrack-shaped depression on the target surface. The rest of the target surface where sputtering does not substantially occur is herein referred to as nonsputtered region(s). For a nonrotating cylindrical magnetron target, the sputtered region would also typically appear as an oval or racetrack-shaped depression on the target surface, depending upon the particular arrangement of magnets near the target. However as most cylindrical targets are rotated about their cylindrical axis during sputtering, a uniform region of sputtering results over a portion of the length of the target. Remaining nonsputtered regions then typically comprise the distal ends.
As reactive sputtering takes place, often a thin layer of the reacted target material (usually nonconductive) builds up in the nonsputtered regions of the target as well as upon other exposed components inside the chamber. In the example of a silicon target sputtered in an oxygenated environment, a thin layer of silicon dioxide (an insulator) will form over the entire target surface, in both sputtered regions and nonsputtered regions. However, where the layer forms in sputtered regions, it is immediately sputtered off the target surface and ejected back into the sputtering chamber. In this manner, a thin insulating layer eventually builds up on the target surface only in the nonsputtered regions.
When a sputtering gas plasma forms near the insulating layer deposited in the nonsputtered regions of the target surface, sparking through the insulating layer to the target surface may occur. As the sputtering gas plasma builds up charge on the outer surface of the insulating layer, the electrical potential across the insulating layer increases. When this potential equals or exceeds the breakdown voltage across the thickness of the deposited insulating layer, sparking occurs through the insulating layer to the underlying target material. Once a spark develops, target matter is evaporated. This increases the ion density near the spark, which further propagates the spark. Eventually most of the cathode power is dissipated by such sparks, thus resulting in catastrophic sparking as referred to herein.
If the insulating layers were allowed to build up to sufficient thicknesses, eventually the breakdown voltage of the layer would be greater than the maximum voltage encountered in the sputtering environment, and sparking would not occur. However, it is the initial stages of build up of the insulating layer that are most critical since the voltages present in the sputtering environment are sufficient to spark through the thin insulating layer. For some silicon/aluminum targets reactively sputtered in an oxygenated environment, the resulting thin oxide layers which build up on the target surface may begin sparking after only a few hours of operation. When sparking occurs, coating quality is compromised owing to variations in the rate of coating, particularly for sputtering processes which pass a moving substrate beneath the target. Moreover, sparks tend to produce flaking of material off internal coater surfaces, which then land on the substrate and obscure further coating. Sparking, therefore, significantly reduces sputtering efficiency, tends to prematurely destroy sputtering targets, and prevents economical sputter coating over sustained time periods.
Thus, the need exists for a sputtering target which is significantly less susceptible to catastrophic sparking than prior known targets. Methods of reducing catastrophic sparking on target surfaces are also desired. Moreover, methods of producing and using nonsparking targets, and especially of magnetron type, either in planar or cylindrical form, are desired.